Buried pipes, such as pipes used in municipal water systems, lose metal resulting in a reduction in pipe wall thickness, as a result of internal and external corrosion. Sustained wall thickness loss eventually causes the pipe to fail. Such pipes have to be periodically inspected and evaluated for structural adequacy. Accurate information about pipe wall thickness is needed to determine the structural capacity of pipes. Also, monitoring of pipe metal loss is needed to estimate the remaining service life of pipes, which is an important part of maintenance management systems of large pipe networks (e.g., urban water or gas distribution systems). It is therefore desirable to be able to assess the condition of buried pipes in a non-destructive and non-intrusive way, that is without damaging the pipe and without taking it out of service.
One way of determining pipe wall thickness is to obtain pipe coupons (or samples) and then measure the thickness off-site using thickness callipers. Also, wall thickness can be measured using an ultrasonic gauge at locations where the pipe is naturally exposed or intentionally excavated. Both methods provide only a discrete measurement of wall thickness. An alternative method for obtaining a continuous measurement of pipe wall thickness employs ultrasonic or magnetic flux leakage or remote-field eddy current devises which are launched inside pipes using robots (known as pipeline pigs).
Excavating pipes for the purpose of measuring wall thickness is costly, as it requires excavating the pipe at several locations. Also, thickness values based on the coupons may not be statistically representative of the overall condition of the pipe unless a very large number of locations are excavated. Pipeline pigging requires taking the pipe out of service. It is strictly suitable for large diameter pipes with simple geometry (i.e., no abrupt bends). Data acquisition and analysis is very intensive and expensive. The method is used mostly for oil and gas transmission pipelines. It is not suitable for water distribution systems, which are geometrically complex and extensive networks comprised of small-diameter pipes that are usually filled with debris and have no pig launching stations.